Instant startup electronic ballast circuit for a fluorescent lamp mainly includes an Active Power Factor Correction (APFC) circuit, and an inverter with an arrangement such as half-bridge. FIG. 1 is a schematic diagram illustrating an electronic ballast circuit 100 of the prior art. With reference to FIG. 1, the electronic ballast circuit 100 includes a rectifier 110, a Power Factor Correction (PFC) circuit 120 for adjusting a power factor, reservoir capacitors C5 and C6, an inverter 140 and an output circuit 150. The rectifier 110 is connected to an Alternating Current (AC) power supply V1, and converts AC electricity into Direct Current (DC) electricity. The PFC circuit 120 adjusts the power factor of an AC input, so as to obtain a DC bus voltage VBUS across the reservoir capacitors C5 and C6. The inverter 140 receives DC input power from the reservoir capacitors C5 and C6, and converts the DC input power into AC output power. The output circuit 150 receives the AC output power from the inverter 140, so as to supply a current for driving at least one load, e.g. the gas discharge lamp.
The rectifier 110 is a full-bridge rectifier, which is composed of diodes D1-D4 and a filtering capacitor C1, so as to convert the AC electricity inputted from the AC power supply V1 into the DC electricity, e.g. an 110 VAC into 150 VDC or a 220 VAC into 300 VDC.
The PFC circuit 120 performs PFC, i.e. input current shaping, which can be implemented in many different manners. The general manner to implement is to use a boost transformer. As shown in FIG. 1, the PFC circuit 120 includes: a PFC startup circuit for providing a startup voltage used to start an operation of the PFC circuit 120 by receiving an input voltage, which, for example, includes a resistor R1 and a PFC power-supply capacitor C3 connected in series between an output of the rectifier 110 and a grounded terminal; a transformer (e.g. the boost transformer) including a primary winding L2A and a grounded secondary winding L2B; a charge pump circuit which, for example, includes a capacitor C2, a resistor R2, a diode D6 and a voltage stabilizing diode D7, where an end of the charge pump circuit is connected to the secondary winding L2B; a PFC controller U1 (e.g. an integrated circuit chip numbered L6562) having an input VCC and an output GD, where the input VCC thereof receives the startup voltage from the PFC startup circuit and an operation voltage provided by a PFC power supply circuit which is composed of the charge pump circuit and the secondary winding L2B; and a PFC switch Q1, which is coupled between the primary winding L2A and the ground and is controlled by a signal outputted from the output GD of the PFC controller U1. For example, the PFC switch Q1 is a Metal Oxide Semiconductor Field Effect Transistor (MOSFET).
After the ballast is powered up, the input voltage (e.g. a rectified sine wave) charges the PFC power-supply capacitor C3 through the resistor R1. When the voltage across the capacitor C3 reaches a threshold voltage for the operation of the PFC controller U1, the PFC controller U1 begins to operate and outputs a drive signal for turning on the PFC switch Q1. At this time, a high-frequency current begins to flow through the primary winding L2A, and an induced electromotive force is produced on the secondary winding L2B, so as to provide the operation voltage for the PFC controller U1 through the charge pump circuit. The high-frequency current flowing through the primary winding L2A forces a forward biased diode D5 to be conducted, so as to charge the reservoir capacitors C5 and C6. The forward biased diode D5 is used to prevent the current from circulating between the capacitor C1 of the bridge rectifier 110 and the reservoir capacitors C5 and C6 (for storing electricity energy required for lighting of the gas discharge lamp). In the PFC circuit 120, the PFC switch Q1 is turned on for a fixed period of time by the PFC controller U1, or alternatively, once the current flowing through the primary winding L2A reaches a certain value in direct proportion to the input voltage, the PFC switch Q1 is turned off by the PFC controller U1, so as to implement the PFC.
As shown in FIG. 1, the inverter 140 is configured in a half-bridge form comprising two inverter switches Q2 and Q3. The inverter switch Q2 is coupled between a first output and an input of the inverter 140. The inverter switch Q3 is coupled between the first output and the ground of the circuit. The inverter switches Q2 and Q3 are appropriate power switching devices such as NPN bipolar transistors.
When the PFC circuit 120 is started up, the DC bus voltage VBUS charges a startup capacitor C8 of the inverter through a resistor R6. When the voltage across the startup capacitor C8 reaches a breakdown voltage of a voltage breakdown device such as a bidirectional trigger diode DB3 (D16), the bidirectional trigger diode DB3 gets through, a voltage drop thereon decreases quickly, and the charges stored in the startup capacitor C8 are released quickly through the bidirectional trigger diode DB3, so as to provide a startup current for activating the switch Q3, thereby starting a self-oscillation operation of the inverter 140. The DC current charging the startup capacitor C8 through the resistor R6 is released by the high-frequency switch Q3 through a diode D15, so as to avoid it disturbs the normal operation of the half bridge by charging again and triggering the bidirectional trigger diode DB3. Hereto, the ballast completes the startup, and begins normal operation.
During the normal operation, the switches Q2 and Q3 are turned on and off alternately by a drive signal, i.e. the switch Q3 is turned off when the switch Q2 is turned on, and vice versa. The drive signal is provided by two secondary windings T1C and T1D of an output transformer T1. A high-frequency output voltage of the inverter 140 for igniting the discharge lamp coupled between a first output connection J9 and at least one second output connection J4, J5, J6 and J7 is provided to the output circuit 150 by a primary winding T1B magnetically coupled to a secondary winding T1A.
When the AC power supply V1 is cut off, the half bridge performs damped oscillation due to the gradually decreased voltage across the reservoir capacitors C5 and C6, and stops the oscillation until the driving is insufficient.
There are three problems in the electronic ballast circuit of the prior art.
1. After power is cut off and the apparatus is shut down, the damped oscillation by the half bridge cannot consume the full energy stored in the reservoir capacitors C5 and C6. This leads to the following actions after the stop of the oscillation performed by the half bridge: the startup capacitor C8 is charged again and triggers the bidirectional trigger diode DB3, the inverter 140 is started up again, and part of the discharge lamps is ignited instantaneously and produces twinkling.
2. This circuit operates in a wide voltage range such as 120V to 277V or 347V to 480V. Upon the startup of the circuit, there is a situation that the inverter 140 is started up earlier than the PFC circuit, i.e. a speed of charging the startup capacitor C8 to a threshold voltage for starting the inverter is greater than a startup speed of the PFC circuit. In this case, the inverter 140 is started up before the bus voltage reaches the rated value. As such, the output voltage (across the primary winding T1B) of the output transformer T1 cannot reach the rated value. Thus, the startup time and startup current of the discharge lamp cannot meet requirements in ANSI82.11, which has an adverse effect on the discharge lamp's life.
3. For the PFC circuit of this type, there exists an issue of output voltage overshooting (the output voltage exceeds the rated value for a short period of time) upon the startup or when the load changes. If the inverter 140 is started up during the time of the bus voltage overshooting, a voltage stress on the inverter switches Q2 and Q3 will also increase considerably and even exceed the rated value, which also bring an adverse effect on the switches Q2 and Q3's service life.